The requirement to provide accurate positional control of an object with respect to some other object or reference surface is encountered in a wide variety of applications. These include applications in which it is desirable, or necessary, to achieve accurate positional control on a small scale, such as a nanometer scale or even finer. Examples of applications requiring fine positional control include the positioning of read/write heads with respect to storage media, the positioning of microscope probes or tips with respect to samples and sample surfaces, sample positioning for microscopy or processing, and the manipulation of tools for working on, or handling, single biological cells or small groups of cells. It is known to use piezoelectric materials in positioning and manipulation apparatus, exploiting the small dimensional changes of the materials in response to applied electric fields.
In a number of applications it is desirable to have a positioning system that can provide both a relatively wide range of overall movement (e.g. on the centimeter scale) and yet fine positional control within that range (e.g. a resolution on the micro or nanometer scale, or better). One known type of such a positioning system is illustrated in FIG. 1. This apparatus comprises a stack of three piezoelectric translators, each of which can be regarded as a type of linear motor and has a base member and a movable member having one degree of freedom. Each translator is controllable to provide “coarse” movement (in this example coarse movement is to be considered as movement in the range 1 micron to 1 centimeter) of its movable member in one of the three orthogonal directions (the nominal X, Y and Z directions in the figure). This movement (i.e. translation in one dimension) is achieved using a technique that is known generally as “slip-stick”. In the slip-stick technique the base comprises a piezoelectric member and a surface of the movable member is clamped against a surface of the piezoelectric member by suitable clamping means. The clamping means may be adjustable so as to adjust the friction force. The piezoelectric member is then controlled, with a suitable control voltage, such that its surface in contact with the movable member performs sawtooth-like oscillations along the direction of intended translation, i.e. it undergoes a series of slow movements in the direction of intended translation, with rapid movements in the opposite direction in between. If the clamping force is suitably arranged then during the slow movements the two surfaces “stick” and the movable member is moved, but during the fast movements there is slippage and the movable member is not moved. Thus, the asymmetric oscillations, which cause the slipping in one direction and sticking in the opposite direction, result in translation of the movable member. Large translations can thus be built up incrementally from the relatively small translations in each “stick” part of the slip-stick cycle. In FIG. 1 the first translator, or stage, XC provides coarse movement control of its movable member in the X direction. The second translator YC has its base mounted on the first stage's movable member, and provides independent coarse movement control of its own movable member in the Y direction. In the same way, the third stage ZC is mounted on the second stage to provide coarse Z movement control. The interface between the base and movable portions of the third translator is thus vertical, and a disadvantage with the illustrated apparatus is that, as a result of this vertical interface, the weight of any further structure carried by the third stage is limited. To achieve fine positional control, in addition to coarse X, Y and Z control achievable with the three, stacked translators, the last translator ZC carries a device for fine movement on the 1 nanometer—1 micron scale. This fine movement device is a piezoelectric tube capable of 3 degrees of freedom movement (XYZfine). It is known for the tube to be provided with an electrode arrangement such as that shown in FIG. 8 (and which is also suitable for use in embodiments of the invention for the same purpose, i.e. to enable fine positional control in three directions). In such an arrangement the tube has electrodes on an outer and on an inner side, and the outer electrodes are segmented to form four independently drivable sectors on the tube. Fine movement control in the Z direction can be achieved by controlling a voltage applied to the outer electrodes in common, relative to the inner electrode, to produce longitudinal extension or shrinkage of the tube. Fine movement of the tube upper end surface in the X direction can be achieved by applying a potential difference between one opposing pair of outer electrodes (to bend the tube, as the material under one electrode is elongated and that under the opposite electrode shrinks with respect to the longitudinal axis). Similarly, to produce fine displacement control of the end surface in the Y direction, a differential voltage may be applied between the other pair of outer electrodes.
Although the arrangement of FIG. 1, comprising three coarse translators and a piezoelectric tube, does provide both fine and coarse movement control in the X, Y and Z axes, there are a number of disadvantages associated with it. For example, the large number of components leads to the unit being expensive, and increases the probability of some failure occurring. Secondly, the assembly has a relatively large size and mass, which can cause refrigeration problems in certain applications where positioning or manipulation at low temperatures is required; the large size means that a correspondingly large volume must be cooled, and then held at low temperature, the large mass means that a relatively large quantity of heat must be removed to lower the positioning apparatus to the required temperature. The large mass also means that changes from one stable temperature to another desired stable temperature can only be achieved slowly. The arrangement of the tube on a stack of three translators is also highly susceptible to external vibrations. The relatively large size furthermore results in the arrangement exhibiting mechanical (acoustical) noise susceptibility. Thermal drifts are also a problem, as are the limited accessibility of manipulator and the limitations on applications for the manipulator resulting from its relatively large overall size.
It is therefore an object of certain embodiments of the invention to provide positioning apparatus (which term will be understood to include manipulators) that obviates or mitigates at least one of the above-mentioned problems associated with the prior art.